One of the continuing problems in the refinery when using catalytic cracking processes is handling the very large amounts of gas produced. Catalytic cracking, and especially fluid catalytic cracking (FCC), is widely used in petroleum refineries in the United States. Refiners in the United States have more capacity for catalytic cracking than for any other single process except distillation. Those refiners have the capacity to catalytically crack over 6,100,000 barrels of oil per day (Oil and Gas Journal, Mar. 24, 1980). Since catalytic cracking is a nonhydrogenative process, it can be appreciated that huge amounts of olefinic gases are produced. Whenever the severity of a catalytic cracker is increased or the feed throughput is increased, even more olefinic gases are produced.
Recovering these enormous amounts of gas for further reaction requires large capital outlays for compressors and gas handling equipment. The alternative is to burn the olefinic gases as fuel for other parts of the refinery or as waste. Unfortunately, because the quantities of gas are huge and capital costs high, these gases are too often burned instead of recovered and reacted.
It can be appreciated that there is a highly intensive search for efficient, economical processes which would allow these reactive olefinic gases to be used further as chemicals, rather than to be wasted, but which do not require massive capital expenditures. This search for more efficient methods of using olefinic gases has continued for some time. U.S. Pat. No. 2,678,904, Kearby et al., May 18, 1954, for example, discloses polymerizing the olefins present in catalytic cracker effluent using a fluidized reaction bed containing an amorphous silica-alumina cracking catalyst. Other methods, using zeolites, have been proposed for processing heavier materials. U.S. Pat. No. 4,006,531, Owen et al., Jan. 3, 1978, discloses preparing the aromatic benzene, toluene, and the xylenes, from heavy reformates using zeolites and fluidized beds. U.S. Pat. No. 4,090,949, Owen et al., May 23, 1978, discloses upgrading poor quality olefinic gasolines by converting them in the presence of carbon/hydrogen contributing fragments using zeolites and fluidized beds. A recent disclosure relates to a combination process for catalytically cracking gas oils and upgrading the C.sub.6.sup.- products using the same dual component zeolite catalyst in both steps. U.S. Pat. No. 4,032,432, Owen, June 28, 1977.
I have discovered that certain silicaceous crystalline molecular sieves can be used in a fluidized reaction zone to produce substantial amounts of olefin oligomers from the normally gaseous olefins produced by catalytic cracking. By oligomerizing all or part of the olefins, the volume of gas that needs to be handled is greatly decreased, plus, substantial amounts of more useful higher molecular weight olefin compounds are produced. Further, the fluidized oligomerization process can operate at the same pressure as the FCC reactor; the need for compressors is greatly lessened. This highly significant advantage occurs even if only part of the gas stream is fed to the oligomerization reactor. Importantly, because oligomerization is highly exothermic, the heat generated by the oligomerization reaction can be used to heat the feed to the oligomerization zone; the process is energy efficient by saving fuel as well as by saving the electricity which would be required for the compressors.
Additionally, the catalysts used are highly stable and the reaction conditions are mild so that the catalyst charge to the oligomerization zone can have a very long service life merely by periodically, or continuously, stripping the charge with hot gas. The necessity of burning to remove coke is greatly lowered.
All of these advantages are achieved by fluidized oligomerization using essentially aluminum-free intermediate pore size silicaceous crystalline molecular sieve catalysts.